Fix defect in choice of authority for signature base string

The spec <http://tools.ietf.org/html/rfc5849#section-3.4.1.2> indicates
that:

    The host and port values MUST match the content of the HTTP
    request "Host" header field.

Fixes https://github.com/idan/oauthlib/issues/142
Update, comment on, and test the implementation.
Make normalize_base_string_uri handle scheme and path more conformantly

* only accept absolute URIs including a scheme
* use the root path if the input path is empty
Merge pull request #143 from matthewlmcclure/idan/142

Fix defect in choice of authority for signature base string